When a transformer having a center tap (CT) is used, for example, the number of switching elements of a power conversion circuit can be reduced. In a transformer, as shown in FIG. 1, a plurality of coils are formed by winding electric wires on a bobbin 11. The ends of the coils are connected to pin terminals 12 of the bobbin 11 to serve as input/output terminals. FIG. 2 shows a typical graphical symbol for a transformer having a CT. The terminal numbers coincide with those the circled terminal numbers shown in FIG. 1.
To form a CT, at least two coils must be formed. This leads to an increase in number of steps in the winding process and a decrease in workability. When one end of each coil is connected to a common pin terminal in order to form the CT, the workability decreases more. The increase in number of steps and the decrease in workability increase the production cost of the transformer, and make it difficult to automate the production of the transformer.
In a transformer which deals with a large current, sometimes a plurality of coils are formed and used as they are connected parallel to each other, so that the resistance of the coils may be decreased. To form a CT, at least two coils are required. To form coils of a transformer having a CT to be parallel to each other, coils in a number twice that of the coils of a transformer having no CT must be formed by winding. This further increases the production cost of the transformer and makes it further difficult to automate the production of the transformer.
Japanese Patent Laid-Open No. 2001-155933 discloses a transformer that uses a plate-like coil on which a plate-like conductor (to be referred to as a “conductive plate” hereinafter) is wound, in order to reduce the number of producing steps of the transformer that deals with a large current. As the conductive plate is formed by pressing or the like, a plate material loss occurs easily when forming the conductive plate by punching. Also, the conductive plate is difficult to machine when compared to an ordinary electric wire. Thus, a plate-like coil is not applied to a transformer with a CT in which the coil has a complicated arrangement.